Well, look, just theoretically:
I think using magic can cause physical deformities in a witch, corresponding to the power of the spells.
(For example, how Agnes turns pale, turns gray, and stops growing because she trained intensively in magic since childhood.)
(But Maeve, because of her belief that magic should only be used when absolutely necessary, actually had a positive effect on her, and she grew so tall.)
It's thought?
